1. Field of the Invention
The present invention relates to a piezoelectric actuator and to a lithographic projection apparatus.
2. Description of Related Art
The term “patterning structure” as here employed should be broadly interpreted as referring to a structure that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure includes:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.        A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using a suitable electronic structure. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.        A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
A lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
To reduce the size of features that can be imaged, it is desirable to reduce the wavelength of the projection beam of radiation. It has been proposed to use wavelengths of less than about 200 nm, for example 193 nm, 157 nm or 126 nm. Further reductions in the wavelength to the range of EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of 5–20 nm) are envisaged. EUV radiation in particular is more conveniently focused and controlled by reflective optics, such as mirrors. However, mirrors in lithography apparatus must be positioned to especially high accuracy, as compared to refractive elements, because any rotational orientation errors are magnified by the total downstream optical path length. In any apparatus using very short wavelength radiation, the optical path length may be of the order of 2 m or more.
For example, to have a good overlay performance, it can be necessary to keep the position of an image of an irradiated portion of the mask stable at a given position at substrate level with an error (e) of less than about 1 nm in particular when EUV is used. If the distance between the mirror and the substrate is 2 m the maximum permissible rotational error of the reflected beam, to keep the system within specification, is 28×10−9 degrees (1×10−9 m/2 m=tan(28×10−9 degrees)), if e=1 nm. Since, for a mirror, the angle of reflection equals the angle of incidence, a rotational error in the position of the mirror will give rise to twice as large an error in the direction of the reflected beam. Thus, the mirror must be positioned with an accuracy of 14×10−9 degrees or better. If the mirror has a width of order 0.1 m and a rotating point at one side, that rotating point must be positioned to within 0.024 nm (tan 14×10−9×0.1=2.4×10−11). Clearly, the accuracy with which such a mirror must be orientated is extremely high and will only increase as the specification for image accuracy increases. The accuracy requirements for position in X, Y and Z are less demanding, as such errors are magnified less at substrate level, but still remain high.
A projection system for a scanning EUV lithographic projection apparatus may include six mirrors, for example, for reflecting and thereby projecting the patterned beam onto a target portion of the substrate. In this case, the mirrors are to be positioned relative to each other with an accuracy of about 0.1 nm. It has been proposed before to use a plurality of one dimensional actuators for adjusting the position and/or orientation of a reflective optical element. For example, a corresponding arrangement is described in EP 1107068 A2. This document describes the use of position sensors to maintain the reflective element stationary in spite of vibrations that might occur. In particular, the actuators or other components of the lithographic projection apparatus, such as a gravity compensator, might cause such vibrations.
A reflective or refractive optical member has six independent degrees of freedom (DOF)—three transitional and three rotational. One possibility to adjust the optical member with respect to more than one DOF is to use a plurality of actuators. The actuators may be piezoelectric, electro-resistive or magneto-resistive and act, for example, perpendicularly to a surface of the optical member which extends transversely to the beam of radiation incident at the optical member. In the past a plurality of one degree of freedom inch-worms have been used to position the reflective or refractive optical member. In a typical one-dimensional inch-worm actuator four piezoelectric stacks (two opposed pairs) surround a central cylinder which is connected to an actuation rod comprising decoupling portions. Each piezo stack comprises 2 layers of which one is capable of expanding/contracting and the other is capable of shearing in one direction. The central cylinder and the actuation rod which is connected to e.g. a central pin (which in turn is connected to the optical member) can be moved only in the axial direction. These one-dimensional actuators themselves may surround a central pin in opposed pairs to make the arrangement more robust. Breakage of a decoupling portion in one of the one-dimensional actuators leads to failure of the whole actuator. The reason that each one-dimensional actuator must be opposed by another one-dimensional actuator is that a non-symmetric arrangement can lead to over-burdening of a single decoupling portion in an actuator. This requirement makes the whole actuator quite large. In order to provide the optical member with the necessary six degrees of freedom, twelve one-dimensional inch-worm actuators are used in a hexapod arrangement. This results in an eccentric construction that takes up considerable space within the lithographic apparatus.